Fish Diseases and Management

4.3.1.Metazoan diseases of fish

4.3.1.1 Flukes - Monogenean parasites: Dactylogyrus and Gyrodactylus: The infection is characterised by cloudiness of skin, grey white colour, focal haemorrhages on skin. The infection is ectoparasitic and can cause considerable damage to the host when present in high numbers. These parasites possess a multiple hooked attachment organ called an opisthaptor which disrupts the integrity of the host's skin and mucus membranes. Crowding and other stress factors predispose tropical fish to monogenean trematode problems. Most freshwater monogeneans can be killed quickly with a 3 to 5 minute saltwater bath (30-35 ppt). Glacial acetic acid or hydrogen peroxide dips will also kill these parasites. Praziquantel baths have also proved to be effective in killing some monogenean worms. While expensive, this is a relatively safe treatment when used at a concentration of 10 ppm for 3 to 6 hours.

4.3.1.2. Lice – Argulus: This is an important crustacean ectoparasite infecting fish. Argulus is a branchiuran crustacean  commonly called the "Fish Louse." Fish lice are flattened creatures with a very distinctive shape and appearance. They move about the skin of a fish very effectively and camouflage themselves well on the host. They suck body fluids from the fish via a sharp stiletto that actually injects a small amount of toxin into the fish. Argulus also possesses a life cycle with pelagic larval stages so the entire aquarium system may have to be treated with organophosphates to control the disease.

4.3.1.3. Anchor worm – Lernaea: The parasite infects large scaled freshwater tropical and temperate species of fish. This parasite possesses a life cycle that includes microscopic pelagic larval stages that molt and grow several times before attacking the fish host. On the host the female anchor worm matures and produces two large egg sacs containing hundreds of Lernaea eggs. This parasite is easily visible to the naked eye and may be more than 2 cm in length. The attachment organ, which resembles an anchor is buried in the host's musculature and allows for the invasion of pathogenic bacteria. Plucking the parasites from the fish is warranted and usually results in inflamed areas which heal quickly. Organophosphates and glacial acetic acid dips are successful in treating the problem. Treatment can also be done with a 10 to 30 minute bath in 10 mg/l of potassium permanganate.